1. Field of the Invention
The present invention generally concerns the improved drive, or transport, of webs between opposed rollers. The present invention particularly concerns a straight transport between opposed rollers of a paper, metal or film media, including large film plates, within a laser imager. During the improved transport undesirable steering and skew, and susceptibility to mechanical tolerances and adjustments, are beneficially reduced.
2. Description of the Prior Art
Many materials processing and handling tasks require driving a continuous roll or web of a material between rollers. Such tasks include the movement of paper, metal or film material between rollers in film imagesetters and platesetters used in the graphic arts.
Particularly during transport of a paper, metal or film media in imagesetters and platesetters, each medium must typically be driven as straight as possible, with the axis of web travel continuously perpendicular to the rollers' axis. In this manner images may be accurately positionally recorded on the medium.
The state of the art in the drive of webs of paper, metal or film within laser imagesetters and platesetters is a pair of opposed rollers consisting of a driven roller and an idler roller. Both rollers are typically made of a material with a high coefficient of friction such elastomeric synthetic rubber. One roller is typically located above the web material and the other below.
In this arrangement several mechanical relationships and alignments are very critical. If appropriate relationships and alignments are not maintained then the web that is driven between the rollers will tend to be steered, or skewed, from off its required straight path of travel, wasting the web and disrupting production. Each roller must be a cylinder rotating about a central axis. Even the slightest imperfection or contamination on the surfaces of the rollers tends to degrade both these requirements, causing that the cylindrical roller should effectively exhibit a diameter which is, at least regionally, both (i) non-uniform and, simultaneously, (ii) slightly eccentric about its axis of rotation.
It is further useful that the speed of the web should be maintained as uniform as is possible. This is because, in a laser imagesetter or platesetter, the medium of the web is being imaged in and upon successive image lines by a scanning light beam. The successive image lines are typically very close together, as many as 1200 per inch and more. The human eye is very sensitive to variations in grey tones, and image discontinuities, that result when successive lines, or regions of lines, are either relatively closer together, or relatively further apart, than are other line regions.
Many strategies exist to maintain the medium in constant uniform motion past the laser marking station of the imagesetter or platesetter, thereby to image the successive lines at a constant and uniform separation. The strategies may involve a high inertial mass in the drive mechanism relative to the mass of the moved, and imaged, web. Alternatively, a dynamic feedback control of the positional drive of the web may be employed. Regardless of the means by which precisely uniform motion is imparted to the web, the ultimate drive of the web is by frictional contact between the web and each of two opposed rollers. If the friction between the web and either, or both, of the rollers varies ever so slightly during the course of imaging--which may be common with minute changes in temperature, vibration, air drafts, contamination, etc.--then minute variations in speed may be imparted to the web. Nonetheless that these speed variations are minute, they are visually detectable in high quality images, and are thus undesirable.
Opposed rollers for web drive must be maintained substantially parallel, and, also, at a distance of separation appropriate to produce a desired compressive force against the web. The compressive force must typically be adjusted in order to account for (i) variations in the friction presented by surface(s) of the two sides of the medium and/or the two rollers, and/or for (ii) for wear in the bearings and/or the drive mechanism of either or both rollers. The adjustment of compression force is accomplished by mechanically adjusting and aligning the rollers, relative to a frame and to each other, in both (i) parallelism and (ii) separation.
The required alignment is commonly accomplished by positioning at least one end of each roller in two spatial dimensions. This adjustment is, in accordance with the tolerances and required exactitude of any particular system, typically fairly complex and intricate. An initial adjustment of the (i) parallelism and (ii) spacing of the rollers is typically made in accordance with visual observations and/or by the use of gauges. Then, an initial coarse adjustment having been made, the actual result of the roller spacing and alignment on web transport is typically visually observed. Further, fine, adjustments are typically required to be, and are, made in response to empirical observations. The entire procedure of successive observations and adjustments is time consuming. The quality of the ultimate alignment is uncertain, and strongly dependent upon the skill of the technician performing the adjustments and observations. The transport rollers may--depending upon the sensitivity of the system to variations, the stability of variables, and/or the required exactitude of web movement--have to be re-aligned undesirably often.
Accordingly, it would be desirable, at least in the instances of the movement of paper, metal or film webs between rollers in film imagesetters and platesetters, if any of the quality, reliability, constancy and/or maintainability of the web transport could be improved. The web would desirably be driven in a highly exact, straight, path for lengthy periods of time during such hard use of the transport rollers as induced normal wear, tear, and variation. During movement of the web the speed of movement would desirably be maintained exactingly constant. If and when alignment and/or adjustment of the web transport rollers were to be required, it would be useful if such alignments and/or adjustments were readily and easily accomplishable to a uniform accuracy and effect.
Meanwhile that the movement of paper, metal or film webs between rollers in film imagesetters and platesetters might desirably be improved, it is known that webs may be transported between opposed rollers that differ in any of innumerable characteristics. Each one of opposite rollers may be, for example, possessed of surface characteristics as suit the processing of the web surface with which it comes into contact. Such processing may include printing or embossing. It might even be hypothesized that some characteristic or characteristics of the oppositely-disposed transport rollers might facilitate, or improve, their function in the uniform, reliable and/or maintainable transport of the web. However, even if it is postulated that one or more surfaces of two opposed rollers should facilitate the transport of a web therebetween, it is uncertain how this (these) roller's(s') surface(s) should work for the particular case of a paper, metal or film medium in a film imagesetter or a platesetter. Where in the transport path should the roller(s) of improved characteristics be positioned? Where should it (they) be positioned relative to the two sides of the paper, or film? Should either, or both, rollers be adjustable, and how should it (they) be so adjusted? If frictional contact between the web and the rollers is to be maintained uniform in order that drive velocity of the web may be maintained highly constant, then how is this to be done?
Because the solution presented by the present invention to the long-persisting difficulties of transport of paper, metal or film material between rollers in film imagesetters and platesetters will be seen to be elegant, straightforward, and highly effective, there may well be a tendency, as with all simple improvements that work well, to denigrate the erudition of the solution and/or the significance of the improvement obtained thereby. However, one rebuttal to such an hypothesized diminution of the stature of the present invention might be to provide a proverbial routineer in the art with (i) an imagesetter or platesetter and (ii) a great box of rollers of diverse characteristics, a so-called box of the "prior art". Although it might well be imagined that the use in the imagesetter or platesetter of rollers of differing characteristics might have some effect upon the transport of the paper, metal or film web, it is entirely unclear as to just exactly what should be done where and how, and to what effect, in order to ensure a reliable and straight transport of the web with less susceptibility to mechanical tolerances than was presented by previous methods.